1. Technical Field
The presently disclosed subject matter relates to a method for producing a color-converting light-emitting device having a light-color converting material layer that is formed on a surface of a light-emitting element and converts the color of light emitted from the light-emitting element.
2. Description of the Related Art
Optical devices are known that include a light-color converting material (for example, a phosphor) for converting the blue light of a light-emitting diode (LED) into red and green light to emit white light as a combination of red, green and blue light. One such known optical device includes a blue LED chip arranged within a cup having a reflective inner surface. The inner space of the cup around the LED chip is filled with a phosphor.
In some cases, uneven colors may be caused by the formation of uneven phosphor layer around the LED chip. Japanese Patent Application Laid-Open No. 2003-69086 (corresponding to U.S. Pat. No. 6,576,488 which is hereby incorporated in its entirety by reference) describes a technique in which a phosphor layer of uniform thickness is formed and adhered on the surface of an LED chip by electrophoresis in order to reduce the occurrence of uneven colors. In this technique, phosphor particles, and aluminum nitride serving as both a charging agent and a binder are dispersed in a solution composed mainly of isopropyl alcohol and water. In the electrophoresis process, an LED chip is placed in this solution along with its support member (submount) that also serves as a cathode. An anode is also arranged opposite to the cathode. An electric field is then applied between the thusly arranged cathode and anode, causing the phosphor particles, which are positively charged by the charging agent, to be electrophoresed (moved) toward the cathode and deposited on the conductive part of the LED chip surface to form a phosphor layer.
As described above, the LED chip and its support member are immersed in the solution during the electrophoresis process and a voltage is applied to them. As a result, the elements present in the LED chip and the support member may dissolve into the solution by ion migration when specific conditions are met. The specific conditions mean that two or more regions of the LED and the support member contain the same element selected from Ag, Pb, Cu, Sn, and Ni, the elements known for their tendency to undergo ion migration, and an electrical potential difference exists between these regions. When these conditions are met, the element in a high potential region is ionized and dissolved in the solution and the dissolved ions migrate to a lower potential region where they deposit, a phenomenon known as ion migration. Of the elements described above, Ag has a particularly high tendency to undergo ion migration.
These elements/materials are conductive and can cause short circuits when they deposit between the adjacent electrode patterns, lowering the reliability of the device. Similar to the phosphor particles, the dissolved ions are positively charged and thus electrophoresed to the lower potential surface of the LED chip where they deposit. The crystallized element form fine particles smaller than the visible wavelength in size. These particles form black-colored light-absorbing spots in the phosphor layer, resulting in a decrease in the luminance of the LED device.